I. Field
The present invention relates generally to communication, and more specifically to techniques for transmitting pilot and signaling in a multiple-input multiple-output (MIMO) communication system.
II. Background
A MIMO system employs multiple (T) transmit antennas at a transmitting entity and multiple (R) receive antennas at a receiving entity for data transmission. A MIMO channel formed by the T transmit antennas and R receive antennas may be decomposed into S spatial channels, where S≦min {T, R}. The S spatial channels may be used to transmit data in parallel to achieve higher throughput and/or redundantly to achieve greater reliability.
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (K) orthogonal frequency subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data. Up to K modulation symbols may be sent on the K subbands in each symbol period.
A MIMO-OFDM system is a MIMO system that utilizes OFDM. The MIMO-OFDM system has S spatial channels for each of the K subbands. Each spatial channel of each subband may be called a “transmission channel” and may be used to transmit one modulation symbol in each symbol period. Each transmission channel may experience various deleterious channel conditions such as, e.g., fading, multipath, and interference effects. The S·K transmission channels of the MIMO channel may also experience different channel conditions and may be associated with different complex gains and signal-to-noise-and-interference ratios (SNRs).
To achieve high performance, it is often necessary to characterize the MIMO channel. For example, the transmitting entity may need an estimate of the MIMO channel response to perform spatial processing (described below) in order to transmit data to the receiving entity. The receiving entity typically needs an estimate of the MIMO channel response to perform receiver spatial processing on signals received from the transmitting entity in order to recover the transmitted data.
The transmitting entity normally transmits a pilot to assist the receiving entity in performing a number of functions. The pilot is typically composed of known modulation symbols that are transmitted in a known manner. The receiving entity may use the pilot for channel estimation, timing and frequency acquisition, data detection, and so on. Since the pilot represents overhead in the system, it is desirable to minimize the amount of system resources used to transmit the pilot. The system may thus employ a pilot structure that provides an adequate amount of pilot for most receiving entities under normal (or most) channel conditions. However, this pilot structure may be inadequate for certain receiving entities observing adverse channel conditions.
There is therefore a need in the art for techniques to transmit pilot for various channel conditions.